(function(doc, win){
    var docEl = doc.documentElement,
    resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
    recalc = function() {
        var clientWidth = docEl.clientWidth;
        if(!clientWidth)return;
        if(clientWidth>750){clientWidth = 750;}
        docEl.style.fontSize = 40 * (clientWidth / 750) + 'px';
    }
    if(!doc.addEventListener) return;
    win.addEventListener(resizeEvt, recalc, false);
    doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window)